In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities, there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and size of memory cells, and surface geometry of various features such as corners and edges.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist, and an exposing source (such as optical light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The photoresist receives a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the photoresist. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the photoresist through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the photoresist as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. A recognized way of reducing the feature size of circuit elements is to lithographically image the features with radiation of a shorter wavelength. "Long" or "soft" x-rays (extreme ultraviolet (EUV), deep ultraviolet (DUV)), wavelength range of 5-200 nm are now at the forefront of research in an effort to achieve the smaller desired feature sizes.
Although EUV lithography provides substantial advantages with respect to achieving high resolution patterning, the shorter wavelength radiation is highly absorbed by the photoresist material. Consequently, the penetration depth of the radiation into the photoresist is limited. The limited penetration depth of the shorter wavelength radiation requires the use of ultra-thin photoresists so that the radiation can penetrate the entire depth of the photoresist in order to effect patterning thereof. However, the thinness of such ultra-thin photoresists results in the etch resistance thereof being relatively low. In other words, the etch protection afforded by ultra-thin photoresists is limited which in turn limits the EUV lithographic process.